Voltage Drop Calculator

Calculate voltage drop in an AC or DC circuit given wire gauge, voltage, current, and length. Determine the correct size for a circuit including the minimum wire gauge and maximum conductor length given an allowable voltage drop.

Calculate Maximum Conductor Length

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What is Voltage Drop

Voltage drop is the amount of voltage lost in a circuit due to resistance of the conductor. Voltage drop is an important consideration when planning a circuit to allow equipment using the circuit to run as designed. Excessive voltage drop could result in damage to equipment and devices or excess heat introducing in a fire hazard.

How to Calculate Voltage Drop

“M” = phase multiplier: Use 2 for a single phase or DC circuit and 3, or 1.732, for a three phase circuit.

“K” = direct current constant: Use 12.9 for a copper conductor and 21.2 for an aluminum conductor. This is equal to the resistance of a conductor that is one thousand circular mils and one thousand feet in length.

“I” = current: This is the current of the circuit in amps. Use our Ohm’s Law calculator to convert from watts to amps.

“L” = length in feet: This is the one-way length of the conductor in feet. Use our length conversion calculators to convert from metric measurements to feet.

“CM” = cross-sectional area: This is the cross-sectional area of the conductor in circular mils. Use our wire gauge calculator to find the area of a conductor in kcmil. To convert kcmil to circular mils multiply kcmil × 1000.

How to Estimate the Conductor Size Needed for a Circuit

Using the equation for voltage drop and a little algebra the minimum conductor size in circular mils for a circuit can be found using the following:circular mils CM = (L × M × K × I) ÷ voltage dropkcmil = CM ÷ 1,000

Substitute values in the formula to find the cross-sectional area in circular mils, then divide by 1,000 to find the size of the conductor in kcmil required. Use our wire gauge size chart to find the wire gauge with the correct cross-sectional area.